Kamina, Simon and the rest of the Great Gurren Brigade plan to take down Thymilph, one of the Four Divine Generals of the Spiral King, and his massive mobile Gunmen fortress, the Dai-Ganzan.
Their plan involves having Simon using Lagann to take over the Dai-Ganzan the same way he fuses with the Gurren, but an something unexpected causes a great tragedy in the following battle.
The final battle between EmperorGreymon and MagnaGarurumon against Cherubimon begins. Cherubimon absorbs all the DigiCode he has stolen from the Digital World.
Now that he's a regular human again, Koichi decides to join the others. Cherubimon attacks and Koichi can't do anything because he doesn't have spirits.